Design of Riprap

7/26/2019 Design of Riprap

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: 0 :

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: :

r

: .

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:

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VOL l6, No. l

79)

THE INDlAN SOCIETY FOR HYDRAULICS

JOURNAL OF HYDRAULIC ENGINEERING

DESIGN OF RIPRAP FOR PROTECTION AGAINST.

SCOUR AROUND BRIDGE PIER

by

Bhalerao A.R.t, F.ISH andGarde R.

J 1

F.ISH

ABSTRACT

From careful study of literature, it is found that various methods have been

developed for protection of the bed against scour around bridge piers. Use of

appurtenances has found limitations as regards effectiveness, structural design and

limited experience in their use.A layer of riprap coarse, non-cohesive and non-movable

material) around the pier enhances the ability of bed material around the pier to resist

the erosion. On the basis of experimental data collected by theauthor, Worman, Chiew

and others, a method is proposed for the design of rip rap layerto control scouraround

circular bridge piers.

INTR UcTION

In the recent times, efforts have been directed towards development ofmethods to.

reduce or control scour around bridge piers, thereby reducing the cost of bridge pier

foundation. These methods include i) modification of upstream face of the pier ii)

placing additional appurtenances, and iii) use of vanes, piles etc. These methods are

found to reduce scour to the extent 20 to

60

percent. However, very few of these

methods have; been tested on prototype bridges and hence, information about their

performance, their effect on stability of the pier and cost involved have not been

reported.

As indicated by some investigators such as Inglis 1942) , Blench 1956), Laursen

1 956), Hancu 1 971), Galay 1987), Worman 1989), Suzuki 1992)and Chi ew 1995)

scour can be effectively controlled by providing a layer or layers of non movable

riprap around the pier, with or without a filters. Worman 1989) has used this method

effectively on some bridges. However, there is a necessity for collecting additional

data in laboratory and field for checking his method or proposing changes in it if

necessary, using additional data. Hence, this investigation was carried out. As regards

use of riprap for controlling scour, onehas to determine the size, gradation and thickness

of riprap layer for kno characteristics of the pier, flow conditions and bed rnateaL


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DESIGN OF RIPRAP FOR PROTI:CTION AGAINST


VOL. 16, No.1)

TWO APPROACHES

The size of non-movable material in riprap can be determined by either critical

velocity approach or critical shear stress approach. In critical velocity approach, the

size of riprap D) can be obtained relating it to average velocity of flow U), depthof

flow Y) and difference in specific weights of sediment and ater

fy

s if one assumes

that viscosity is not important. Investigators such as Ishbash 1935), Garde 1970),

Bonasoundas 1973), Quazi and Peterson 1973), Maynord et al. 1 989), Richardson

et al. 1991) , Garde and Kothyari 1995) Parola 1995), Chiew 1995) have suggested

equations for computation of size of riprap. Most of these equations are for the

computation of size of riprap in unobstructed flow.Further, it may be noted that when

water flows, shear or velocity distribution around the bridge pier is affected and

instantaneous values of these two parameters vary and much greater than time averaged

values. These two aspects are not considered in these equations.

;

......

Alternately one can specify that /).

D

in case of riprap layer should be less

than a specific value for it to be stable and control scour. Garde and Kothyari 1995)

recommended this value as 0.03 for riprap in a channel. Here

to

is average shear

stress in the channels equal to RS. However, in case of bridge pier, local shear around

the pier is greater than the average shear stress in the channel. Assuming to is

proportional to l.P, experimental evidence indicates that the time averaged local shear

stress around the pier can be 3 to 5 times the average shear stress in the channel.

Measurements of shear stress around pier by Hjorth 1975) andDarghi 1987) indicate

that instantaneous shear around pier can be as high as 11 to 12 times

to.

These two

facts need to be taken in to account while developing the method for design of riprap.

As mentioned by Chiew and Melville 1989) the effect of sediment gradation is

negligible when standard deviation

Jg

of riprap is less than 2. Hence, it is desirable

to have standard deviation

J

g of riprap between 2 and 3. Filter underneath the riprap

layer is usually required to prevent leaching of base material, which takes place due

to penetration of turbulence in the riprap layer. This takes place due to penetration of

turbulence in the riprap layer. However, considering the difficulties in laying the filter

layer, Worman 1989) has indicated that two or more layers of graded riprap can be

designee in such a way that provision of filter is not needed. This approach is adopted

here.

-.

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16.2010, NO.1

< ;> .

.

BRIEF REVIEW

Generally, the size of riprap is determined by using one of the equations available

for critical velocity. Worman 1989) has used Ishbash 1935) equation in which

diameter of riprap D is expressed as function of critical velocity as given below.


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VOL. 16, (No.1)

DESIGN OF RIPRAP FOR PROTECTION AGAINST


(81 )

l .

u,

=O .8 sl2g p ;p

D

Hereu =2U.

c

(1)

j

t

l.

r

On the basis of small-scale experiments, Suzuki (1992) suggested that the thickness

(T) of the riprap be obtained from Eq.

(1).

(2)

where

r.,

and

t o

are dimensionless critical shear stress given by (

1

J

and

r 5

I

. : -:---

v- :.:

respectively. Kulkarni (1993) has recommended that, thickness of riprap

Y s

D

(T) can be obtained from the equation.

U

2

T=:

g

:;

J

(3)

I

where U is average velocity in unobstructed flow. This equation is to be used for

protection of bed and banks in the river. On the basis of analysis of the laboratory data

collected, Worman (1989) has proposed the following equation for thickness of riprap

as,

...

:.. -

(4)

where T is thickness of riprap, p is coefficient of friction for turbulent flow,

CD

is the

drag coefficient of particle of bed material and n is porosity the value of which is

taken as 0.38. Inserting values of p ,

PS

Pf' Co and n, this equation reduce to

U

6 )

gT

D

I5

If

85

>

0.15

D

I5

(5)

It may be mentioned that in Eqs. (4) and (5), Worman uses U= 2U

o

where U,

average velocity in unobstructed channel, in order to account the fact that scouring

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DESIGN OF RIPRAP FORPROTECTION AGAINST


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No.

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velocity near the bridge pier is greater than average velocity in unobstructed flow.

U

d

Figure 1shows variation

o

T

and

S

when dgs is less than

1

Onthe basis

g IS

of short duration tests, Chiew 1995) plotted the graph of Uc against T 50 and has

shown separate regions in which riprap around bridge pier had failed and in intact

condition. Refer Fig. 2.

Recently some work has been done by Kothyari, Hager and Oliveto 2007 to

predict densimetric particle Froude number at incipient scour condition near bridge

pier as a function of Rld

50

dsidI6) and geometry of obstruction.

EXPERIMENTAL PROGRAMME

Keeping in viewthe information available, the problem posed for the study was to

develop a method to determine size and thickness of the riprap, which willprotect the .

bed around the bridge pier from scour. For the bed material and riprap size chosen,

three types of experiments were conducted in

0.30

m wide,

0.60

m deep and

10

m

long tilting flUII1en the Fluid Mechanics Laboratory of Civil Engineering Department

of Bharati Vidyapeeth University. Experiments were related following conditions

8

: 6

gT

2

0

0

i

O 0.05 0.1 0.15

FIG. 1 THICKNESS OF RIPRAP WORKMAN RELATIONSHIP

0 8

ViDc

0 6

004

0 2

0

F


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.

l

I.

, ;

i -

I

i

t

.,

DESIGN OF RIPRAP FOR PROTECfION AGAINST


1. Incipient Scour of bed material around the pier

2. Incipient scour of riprap of different size and thickness

3. Scour with riprap protection.

Over 150runs were conducted using circular pier of 50 mrn diameter under clear

water condition. Table 1 gives standard deviation, size of bed and riprap material

tested in the experiments. The circular ring with engraved marking innun was used to

lay riprap of appropriate thickness and flush with original bed level, Bed material of

predetermined thickness was removed from scoured area, weighed and riprap of same

weight was then slowly added to the scour hole and levelled to the undisturbed bed

level.

In addition to the data collected in the present study, data collected by Knight

(1975), Dey (1995), Chiew (1995), Melville (1997),ha:ve also been used for

determining DIU

c

for incipient scour and those by Worman (1989) and Chiew (1995)

for size and thickness ofriprap layer. Worman (1989) had used three bed materials of

median diameter 0.17,0.36 and 0.78 mm and five ripraps of size varying from 8to48

mm. Depth of the flow varied from 300 to 400 mm whereas Chiew (1995) used bed

material of mean size 0.96 mm and three riprap of size 2.60, 4 and 4.85 mm.

t : . . . : :

t

I

TABLE-l

CHARACTERISTICS OF BED AND RIPRAP MATERIAL

(pRESENT STUDy)

Bed Material Riprap Material

d

so

(mm)

0

Dso(mm)

O g

0.20

2.45

1

1.37

0.27

2.69 2.

1.90 .

0.36 1.28

3

2.36

0.40

2.53 4 2.50

0.50 2.63 5 2.66

0.68

2.73 8

1.58

.

.

s ;

...

.

ANALYSIS OF DATA

Limiting values ofUfUcfor Incipient Scour

The critical velocity at which sediment of a given size will just move in an

un bstructed uniform flow was obtained by combining Shields andYalin-Karman

relationship with Karman-Prandl s equation of iu in hydro-dynamically smooth

and rough channels. Analyzing the generated data, following type of equation was

obtained.

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL 16,2010, NO. I

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f.J i::S*/:' ':':''i:;;:;:: :::;:;:);:j;:< : % =ti :}: :: :': :-:: $ . .

c

84

DESIGN OF RIPRAPFOR PROTECTION AGAINST

S OUR AROUND BRIDGE PIER

VOL. 16, No. I)

6)

where C is constant, m and n are exponents, which depend on boundary conditions

and R

o

is equal to

y s

d

3

/

f

v

2

Forrough boundaries n is zero. Similarly for transition

region, n is nearly zeroand for smooth boundaries only it is significant, refer Table 2.

TABLE-2

VALVES OF C,

AND

c

Type of Boundary

C m

n

Smooth

1.77

0.166

0.05

Transition

1.38

0.18

0.000018

Rough

1.65

0.18

.

....

Experiments were conducted to find incipient motion condition of non-uniform

sediments near circular bridge pier of 50 nun diameter. Thus magnitude of U/U

c

at

which bed material near circular bridge pier started moving, has been obtained for all

bed materials seven) used in experiments. Similarly data collected by other

investigators have also been analyzed for the samepurpose and presented inthe Table

3. Here Uc is velocity in the channel at which scourjust occurs around the bridge pier

and U is velocity of flow in unobstructed channel. On the basis of these data an

average value ofUlU

c

=

0.43 was obtained.

Therefore, from the Table 3 onecan select average value of U U, as 0.438 for the

incipient motion ofnon uniform bed material near circular bridge pier. Hjorth 1975),

Melville 1975) and Darghi 1987) as stated by Parola 1995) used Preston

measurement for measuring shear stress near the pier. Their results in terms of U/Uc

and corresponding average shear stress near bridge pier

't

are given in Table 3. It is

known that in the unobstructed flowfor hydro-dynamically rough boundaries

'to-U2,

using this relationship as an approximation, one can write that r

=

where M

= =

U/Uc. Stability of riprap stone is directly related to whether the threshold of the

sediment entrainment of the ripraphas been exceeded or not. It is therefore appropriate

to assume if the ratio of undisturbed velocity U) and average critical velocity U) of

the riprap stone is less than 0.43, riprap stone will remain stable. This implies that the

average shear stressnear the pier (t

p

is five times in the comparison with unobstructed

flow, i.e.

7)

.,

ISH JOURNAL OF HYDRAuLIC ENGINEERING, VOL. 16,2010, NO.1


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:.;' __ = ... .. _ . _'-':: =--_''''''': '''' ':''''_'''':''''''-'/:-J';.'.r

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i

i

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. . .

,

: ::: : >r ;p7 : .: -. .-.

. . 0 . . - -.: - : :

VOL. 16, No. I)

DESIGN OF RIPRAPFOR PROTECTION AGAINST

SCOUR AROUND BRID GE PIER

(85)

.\ TABLE-3

UlU

c

AND CORRESPONDING SHEAR AT PIER

S.N

Name of the Investigator

UIU

't

p

=

M

't

oc

Shear Stress by velocity Measurement

I

Nicollet 1977)

0.42

5

Circular PierRounded nosed Pier

0.50-0.65

3

2

Lee Jong 1973)

Circular pier without

0.40 6.25

attachment Round nosed and

0.50 4.00

Rectangular

3 Bressure and Roudkivi 1977)

0.5 4

4

Chiew 1995)

0.3

11

5

Melville

1999)

0.34

8

6

Dey l993)

0.475

4

7

Present

0.438

Shear Stress by Preston Measurement

8

Hjorth 1975)

-

12

9 Melville 1975)

-

3.5

10

Darghi 1987)

-

3.5

.

Studies of Einstein and E1. Sarnni l949), Gessler 1967) and Little Mayer l972)

have shown that the lift as well as shear at the bed fluctuates in turbulent flow, and

follows Gaussian distribution as an approximation with standard deviation

J

in

dimensionless form varying from 0.45 to 0.57. Therefore, the maximum shear stress

near the pier can be t

pmax

Tp 3 x 0.45Tp)

=

2.35t

p

. Here, value of o assumed is

0.45. Therefore, riprap layer can be disturbed when t

pmax

2.35 5[)

=

12 to

Patel and Raga Raju 1999) in their analysis of critical shear stress of non uniform

material have recommended the use of characteristic size of bed material D which

is given by D g x o g instead ofD to for account non- uniformity of sediments. Here,

D

g

isgeometric median size of riprap and

o

g

geometric standard deviation. Therefore,

to prevent scour around the bridge pier , the size of the riprap material D can be

g

calculated as,

D

= 12'[0

fl ys t.

o

8)

SH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16,2010, NO.1


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VOL. 16, No.1)

where n g x g

9)

Asan approximation, for Gaussian distribution Dg and D(J assumed as D50 and D84

of rip rap mixture respectively. Further, Patel and Ranga Raju 1999) have given

t.

co

as a function of g This relationship is used to obtain t.

co

for corresponding given

magnitude of

g of the material used by Worman, Chiew and in present study.For the

determination of size of riprap, one can choose the magnitude of

g

and substitute the

corresponding value of

t.c J

in Eq. 9) and can find average size of riprap.

Size of the riprap calculated using Eq. 9) for Worman data was found to be 50 to

96 percent higher in comparison with size of the riprap used in his experiments.

Worman suggested use ofIshbash equation for the determination of size of the riprap

taking local velocity u) around the pier as twice the average velocity in unobstructed

flow u=2U). Size of the riprap computed using Ishbash equation with u= 2U) for the

data collected by Worman was also found to be 25 to 50 percent higher than those of

used in his experiments.

Further, data collected by Worman and Chiew were analyzed for computation of

non-dimensional critical shear stress

( t.J

for incipient scour of riprap and it was

found that average value of

t.

c

as 0.00174 and 0.013 respectively. In the present

study nms were also conducted for incipient scour of riprap and average value of

non-dimensional critical shear stress

-r.J

for riprap of given size was found to be

0.0088. Further, in the present study 41 runs were observed either with no scour or

with negligible scour. The non-dimensional critical shear stress

-r.J

in these scour

runs was found to vary from 0.003 to 0.09 giving an average value of 0.028. Table 4

gives these details.

TABLE-4

NON-DIMENSIONAL CRITICAL SHEAR STRESS

S.No.

Name of the Investigator

t.,

Jc

1 Worman 1989)

0.00174

1.18 -1.38

2 Cfiiew 1995)

0.136

1.25 -1.27

3

Present- runs for incipient scour of riprap

0.0088

1.36-2.67

analysis)

4 Present - zero scour run with riprap

0.028

1.36 - 2.67

ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16,2010, NO.1

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VOL. 16,(No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST


87

:.

Fromthis table, it is evident thatWorman's method over predicts the size by about

5 to 8times, and Chiew's method gives 2 times larger size than that of observed in the

experiments, where as in case of data collected in the present study it is 1.6 times

(average) larger in comparison with observed size of rip rap.

ANALYSIS O RIPR P TmCKNESS

For studying the effectiveness of riprap in reduction of scour, the parameter C.=

C

a

/C

b

has been calculated. Here C

b

is the value of constant obtained illKothyari et al.

(1992) equation for scour in clear water studies, for non-uniform base material (Eq.

10.. .

10

C is value of C when riprap was used and some scour was observed.

Theratio of C/C

b

called C. takes in to account the effect ofU, opening ratio

a

and

1 Y s

on scour and hence, it should be function of D,=

dsJDso

and T. = T/3cr.

D so

related to rip rap layerand bed material size only. Thickness of riprap layer can be non

dimensionalised by maximum size of the riprap material, which can be expressed as

3.

D so

Anew term therefore, introduced and expressed as T. =T/3cr

a

D

so

where T

is the thickness of the riprap layer, cr is the standard deviation of riprap mixture

a .

given by D84 /D

16

and s ismedian size ofriprap mixture. lfthe sizesin riprap are

distributed normally, 99.73 percent values will bewithin the range ofD so3c . Hence,

3cra Ds is as good as maximum size of the riprap mixture when D100 is not known.

The experimental data having eight ranges of D. starting from 0.045 and 0.78

were plotted as C/C

b

Vs T. for respective range ofDiand the equation between them

is obtained as

-

'-'.-

,:.:

;;..

C

a

_ C

=

0.5 D9 T

2

50

b

(ll)

By assuming that at a value of C. as low as 0.05, riprap around the bridge pier will

be stable. Hence, this equation can be solved for with this value for determining the

thickness of riprap. Data collected by Worman (1989) and Chiew (1995) are used for

the comparison of the thickness of riprap layer computed using Eq. (10). Figure 3

shows the thickness of riprap layer used by these investigators in their experiments

for zero scour condition and thickness computed using Eq. (11). The plot shows 86

of Worman's data points and 68

ofChiew's data points fallwithin the error band of

.

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l ':"'2C":;';{k) "i;,,:dX'6B' k;X J/%#iiMtiiiW;i;l1li .1:i1t {l 'i;';';

>:

#

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.

e :

.:

f:-

':'.:

(88)


SCOURAROUND BRIDGEPIER

VOL 16.(No, 1)

v

50

.

Data collected in no scour runs of present study are also plotted in the same

figure. It is observed that the 75% of data collected in present study fall in the error

band of

50 .

Thickness of riprapprovided should also be economical. Blench (1957) suggested

that thickness of riprap for bed protection may be three times the largest size of the

stone in a mixture

(DI .

Figure 4 shows the comparison between TlD

lo o

observed

and computed.

If Eq. (11) is used to compute the thickness of riprap for Worman's and present

data. thickness of riprapcomputed is found to be orderone to three times D 100; however.

it is four to five times that of D

IO O

in case of datacollected by Chiew, Thickness of

riprap layer using Worman and present data are found appropriate. however, it is

slightly higher in case Chiew's data. In Worman's data the flow conditions, the

characteristics of riprap, bed material and thickness of riprap used were pertaining to

the observed "no scour" condition. In Chiew's data, the flow conditions given were

intended for intact andfailed conditions of riprap. Average of flowdata corresponding

to intact and failed condition has been used in the present analysis to verify the Eq.

(11). Theflow conditions pertaining tocondition ofriprap described as intact. observed

in his experiments may not be corresponding to the true Uno scour" condition. This

could be possible reasonfor computed thickness of riprap found higher than observed.

Line of Agreement.

1000

:

..

.:,

..

.0

.

t

,

t

r

Line of Agreement

'

l

_; 1

I /Cl

B

i

6

.0

.1 11

I-

.

0

1

10

100 1000

AG. 3 THICKNESS OF RIPRAP (PRESENT METHOD)

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8 )

Incipient Scour of Bed and Riprap Material

The value of

c near the circular bridge pier in present study varied from 0.3 to

0.65 and authors recommends it as 0.438. When bed material around the circular

bridge pier starts just moving, the value of UfU

c

is 0.438.

Assuming the to al and corresponding fluctuations in the shear/velocity around

bridge pier, authors found that maximum value of instantaneous shear stress near the

bridge pier is about 12 to. Size of the riprap can be calculated using Eq. (9) with

Jg

of

riprap between 2 to 3.

l S

t p

L .

5

)

I< - .

.

1 1

,,

1

1

1

FIG. 4 COMPUTED VS OBSERVED VALUES OF T l oo (PRESENT METHOD)

CONCLUSIONS

Equation (11) can be used for calculating thickness of riprap of known size and

gradation for given median size of the bed material, around the bridge pier, which

will give nearly zero scour. This above equation gives the thickness of riprap (for data

collected in present study and by other investigators) within the error brand of

50

and maximum thickness of the order of three times D 100' From the safety point of

view, it is recommended to use a factor of safety of order two, in computing thickness

of riprap.

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ACKNOWLEDGEMENTS

The authors are thankful to the reviewers for their constructive comments.

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Flow Structure and Scour Problem atCircular BridgePiers.

Report No.28, O. Miller Institute, Munich Technical University.

Chiew, Y. M. and Melville, B. W. (1989).

Local Scour at Bridge Piers with Non-

Uniform Sediments.

Proc. ofInst. Civ. Engineers, Part 2, 87, pp. 215-224.

Chiew,Y. M. (1995). Mechanics of Riprap Failure at Bridge Piers. JHE, ASCE, Vol.

-116, No.4, pp. 5-529.

Darghi, B. (1990).

Controlling Mechanism ofLocal Scour.

JHE,ASCE, Vol. 116,No.

. 10,pp. 1197-1214 .

Dey S. (1997).

Local Scour at Piers Part I Review of Development of Research.

DSR, Vol. 12,No.2, pp. 2346.

Einstein, H. A. and El Samni, S. A. (1949).

Hydrodynamic Forces on a Rough W ll.

Review of Modem Physics, American Institute of Physics, VoL 21, No.3.

Galay, V.

1

and Quazi M. E. (1987).

River Bed Scour and Construction of Stone

Riprap Protection in Sediment Transport GravelBed Rivers.

John Wiley and Sons

Ltd., pp. 353-382.

Garde, R. 1. (1970).

Initiation ofMotion on Hydro Dynamically Rough Surface Critical

- Velocity Approach.

TIP,CBIP, New Delhi, pp. 271-282.

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Garde, R. J. and Ranga Raju, K. (2000).

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llIrdEdition.

Gessler, J. (1973).

Behavior of Sediment Mixtures in Rivers.

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Symposium on River Mechanics, Bangkok (Thailand) IAHR, pp. A 10-35.

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Sur Le Calcu Des Affouillements Locaux Dans La Zone Des Piles

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Congress ofIAHR, Paris, France, 3,pp. 299-306.

Hjorth, P. (1992).

Studies on Nature of Scour.

Bulletin, SeriesA, No. 46, Institute for

Tknisk Vatternresursla ra, Lund.

Inglis, C. c Thomas, A. R. and Joglekar, D. V. (1942).

The Protection of Bridge

Piers against Scour.

Research Publication No.5, CWPRS, Pune, pp. 35-38.

Johnson, P. A. (1995).

Comparison of Pier Scour Equations using Field Data.

JHE,

ASCE, Vol. 121, No.8, pp. 626-629.

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DESIGN OF RlPRAP FOR PROTECIlON ,AGAINST


91

I

i

i

Knight, D. W. (1975). A Laboratory Study of Local Scour and Bridge Piers. Proc.

XVI Congress of IAHR, Sao, Paulo, Brazil, VoL 2, pp. 243-250.

Kothyari,

V.

C. et al. (1993).

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v

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NOT T ONS

a angle between axis of the pier and approach flow

con stant used by Worman

s

specific weight of sediment


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Documents

Design of Riprap